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McDonald’s customers took to social media to blast the chain for missing what they say is a crucial ingredient in the restaurant’s mozzarella sticks: cheese.

Throughout the month of January, a handful of McDonald’s customers posted photos of what looks like hollow breaded sticks. What is missing from the center is mozzarella cheese, and customers are not happy about it.

According to the Chicago Tribune, McDonald’s apologized Thursday, explaining that the cheese must have melted out during the baking process.

“We are aware of a low volume of guest concerns about our Mozzarella Cheese Sticks,” spokeswoman Lisa McComb said in an emailed statement to the newspaper. “In these instances, we believe the cheese melted out during the baking process in our kitchens and shouldn’t have been served. We apologize to any customers who may have been affected. We are working to fix this in our restaurants.”

The rage and disappointment of customers comes not long after the chain included mozzarella sticks on its menu nationwide. Social media virtually cheered on the restaurant’s decision, and everything was pointing to a successfully executed business move that would boost growth.

McDonald’s has been struggling with growth in recent years. In October last year, it reported its first quarter of growing U.S. sales in two years.  “While still in the early stages, we believe our turnaround plan is starting to generate the change needed to reposition McDonald’s as a modern, progressive burger company,” said CEO Steve Easterbrook, according to the chain’s report.